Titanium alloys and pure titanium, as compared with steels, have comparatively high strength, and are progressively applied to the field of transportation machines including automobiles for which lightening is strongly desired as principal machines. Stainless steels are principal materials for forming an exhaust pipe included in an engine exhaust system. Studies have been made to use titanium exhaust pipes for lightening. Since some parts of an exhaust pipe are heated at a high temperature of 500° C. or above, the exhaust pipe is oxidized rapidly and hence high-temperature oxidation resistance is required to improve durability.
Exhaust pipes included in an engine exhaust system are muffler components including an exhaust manifold, an exhaust pipe, a catalytic muffler, a premuffler, a silencer (main muffler) for an automobile or a motorcycle.
Improvements in titanium alloys has been proposed in addition to various surface treatment processes to improve the high-temperature oxidation resistance (hereinafter, referred to also simply as “oxidation resistance”) of titanium materials. For example, a titanium alloy proposed in Patent document 1 has an Al content between 0.5 and 2.3% by mass and an α phase as principal structure. A titanium alloy proposed in patent document 2 contains Al and Si in an Al content between 0.3 and 1.5% by mass and a Si content between 0.1 and 1.0% by mass. It is mentioned in Patent document 1 that Si suppresses the growth of crystal grains to improve a fatigue characteristic, limits the reduction of corrosion resistance due to the addition of Al to the lowest possible extent, and improves high-temperature oxidation resistance, scale loss resistance and oxygen diffusion phase formation resistance.
Various surface treatment processes for enhancing the oxidation resistance of titanium materials have been proposed. For example, a material proposed in Patent document 3 is formed by cladding a titanium alloy with an Al plate. A plating method proposed in Patent document 4 coats the surface of a titanium alloy with an Al—Ti material by evaporation. A method proposed in Patent document 5 coats the surface of a titanium alloy with a TiCrAlN film by a PVD process.
The cladding method is costly. An evaporation process and a PVD process need a high processing cost and have difficulty in forming an oxidation-resistant film on the inside surface of a tubular titanium workpiece, such as an exhaust pipe.
Patent document 6 proposes a method of forming an oxygen barrier film capable of preventing the diffusion of oxygen into a material, namely, an oxidation-resistant film, by depositing an inorganic binder and Al powder on the inside surface of a material and subjects the material to firing or a processing method that seals pores formed in the Al powder with a sealing material containing chromic acid as a base material after firing. A previously proposed surface-treated titanium material is formed by an inexpensive, safe surface treatment process developed by incorporating improvements into the foregoing method. For example, Patent document 7 proposes a surface-treated titanium material formed by coating a base material of pure titanium or a titanium-base alloy with a fired oxidation resistant layer of a thickness of 5 μm or above and filling up gaps between Al alloy particles and having a Si atomic percent of 10 at. % or below or pure Al with a compound containing one or some metal elements M including Ti, Zr, Cr, Si and Al, C and/or O.
Patent document 8 proposes a method of improving high-temperature oxidation resistance. This method coats the surface of a titanium alloy with an Al-containing layer by hot dipping and seals gaps in the Al-containing layer and nonplated parts by a blasting process using a high-pressure blast of air containing hard particles of alumina, glass or a metal. Patent document 9 proposes forming a protective film processes the surface of an Al-containing titanium alloy material by a shot blasting process using fine particles of molybdenum, niobium, silicon, tantalum, tungsten and chromium to form a protective film in which the particles are dispersed.    Patent document 1: JP 2001-234266 A (Claims)    Patent document 2: JP 2005-290548 A (Claims)    Patent document 3: JP H10-99976 A (Claims)    Patent document 4: JP H6-88208 A (Claims)    Patent document 5: JP H9-256138 A (Claims)    Patent document 6: JP No. 3151713 B (Claims)    Patent document 7: JP 2006-9115 A (Claims)    Patent document 8: JP 2005-36311 A (Specification)    Patent document 9: JP 2005-34581 A (specification)